The present invention relates to aggregating integrated circuits and, in particular, to stacking integrated circuits in chip-scale packages.
A variety of techniques are used to stack packaged integrated circuits. Some methods require special packages, while other techniques stack conventional packages. In some stacks, the leads of the packaged integrated circuits are used to create a stack, while in other systems, added structures such as rails provide all or part of the interconnection between packages. In still other techniques, flexible conductors with certain characteristics are used to selectively interconnect packaged integrated circuits.
The predominant package configuration employed during the past decade has encapsulated an integrated circuit (IC) in a plastic surround typically having a rectangular configuration. The enveloped integrated circuit is connected to the application environment through leads emergent from the edge periphery of the plastic encapsulation. Such xe2x80x9cleaded packagesxe2x80x9d have been the constituent elements most commonly employed by techniques for stacking packaged integrated circuits.
Leaded packages play an important role in electronics, but efforts to miniaturize electronic components and assemblies have driven development of technologies that preserve circuit board surface area. Because leaded packages a have leads emergent from peripheral sides of the package, leaded packages occupy more than a minimal amount of circuit board surface area. Consequently, alternatives to leaded packages have recently gained market share.
One family of alternative packages is identified generally by the term xe2x80x9cchip scale packagingxe2x80x9d or CSP. CSP refers generally to packages that provide connection to an integrated circuit through a set of contacts (often embodied as xe2x80x9cbumpsxe2x80x9d or xe2x80x9cballsxe2x80x9d) arrayed across a major surface of the package. Instead of leads emergent from a peripheral side of the package, contacts are placed on a major surface and typically emerge from the planar bottom surface of the package.
The goal of CSP is to occupy as little area as possible and, preferably, approximately the area of the encapsulated IC. Therefore, CSP leads or contacts do not typically extend beyond the outline perimeter of the package. The absence of xe2x80x9cleadsxe2x80x9d on package sides renders most stacking techniques devised for leaded packages inapplicable for CSP stacking.
CSP has enabled reductions in size and weight parameters for many applications. For example, micro ball grid array (xcexcBGA) for flash and SRAM and wirebond on tape or rigid laminate CSPs for SRAM or EEPROM have been employed in a variety of applications. CSP is a broad category including a variety of packages from near chip scale to die-sized packages such as the die sized ball grid array (DSBGA) recently described in proposed JEDEC standard 95-1 for DSBGA. To meet the continuing demands for cost and form factor reduction with increasing memory capacities, CSP technologies that aggregate integrated circuits in CSP technology have recently been developed. For example, Sharp, Hitachi, Mitsubishi and Intel recently undertook support of what are called the S-CSP specifications for flash and SRAM applications. Those S-CSP specifications describe, however, stacking multiple die within a single chip scale package and do not provide a technology for stacking chip scale packages. Stacking integrated circuits within a single package requires specialized technology that includes reformulation of package internals and significant expense with possible supply chain vulnerabilities.
There are several known techniques for stacking packages articulated in chip scale technology. The assignee of the present invention has developed previous systems for aggregating xcexcBGA packages in space saving topologies. The assignee of the present invention has systems for stacking BGA packages on a DIMM in a RAMBUS environment.
In U.S. Pat. No. 6,205,654 B1 owned by the assignee of the present invention, a system for stacking ball grid array packages that employs lead carriers to extend connectable points out from the packages is described. Other known techniques add structures to a stack of BGA-packaged ICs. Still others aggregate CSPs on a DIMM with angular placement of the packages. Such techniques provide alternatives, but require topologies of added cost and complexity.
U.S. Pat. No. 6,262,895 B1 to Forthun (the xe2x80x9cForthun patentxe2x80x9d) purports to disclose a technique for stacking chip scale packaged ICs. The Forthun patent discloses a xe2x80x9cpackagexe2x80x9d that exhibits a flex circuit wrapped partially about a CSP. The flex circuit is said to have pad arrays on upper and lower surfaces of the flex.
The flex circuit of the Forthun xe2x80x9cpackagexe2x80x9d has a pad array on its upper surface and a pad array centrally located upon its lower surface. On the lower surface of the flex there are third and fourth arrays on opposite sides from the central lower surface pad array. To create the package of Forthun, a CSP contacts the pad array located on the upper surface of the flex circuit. As described in the Forthun patent, the contacts on the lower surface of the CSP are pushed through xe2x80x9cslitsxe2x80x9d in the upper surface pads and advanced through the flex to protrude from the pads of the lower surface array and, therefore, the bottom surface of the package. Thus, the contacts of the CSP serve as the contacts for the package. The sides of the flex are partially wrapped about the CSP to adjacently place the third and fourth pad arrays above the upper major surface of the CSP to create from the combination of the third and fourth pad arrays, a fifth pad array for connection to another such package. Thus, as described in the Forthun disclosure, a stacked module of CSPs created with the described packages will exhibit a flex circuit wrapped about each CSP in the module.
The previous known methods for stacking CSPs apparently have various deficiencies including complex structural arrangements and thermal or high frequency performance issues. Typically, the reliability of chip scale packaging is closely scrutinized. During such reliability evaluations, CSP devices often exhibit temperature cycle performance issues. CSPs are generally directly mounted on a PWB or other platform offset from the PWB by only the height of the ball or bump array emergent from the lower surface of the CSP. Consequently, stresses arising from temperature gradients over time are concentrated in the short lever arm of a low-height ball array. The issues associated with temp cycle performance in single CSPs will likely arise in those prior art CSP stacking solutions where the stack is offset from the PWB or application platform by only the height of the lower CSP ball grid array.
Thermal performance is also a characteristic of importance in CSP stacks. To increase dissipation of heat generated by constituent CSPs, the thermal gradient between the lower CSP and upper CSP in a CSP stack or module should be minimized. Prior art solutions to CSP stacking do not, however, address thermal gradient minimization in disclosed constructions.
What is needed, therefore, is a technique and system for stacking integrated circuits packaged in chip scale technology packaging that provides a thermally efficient, reliable structure that performs well at higher frequencies but does not add excessive height to the stack yet allows production at reasonable cost with readily understood and managed materials and methods.
The present invention stacks chip scale-packaged integrated circuits (CSPs) into modules that conserve PWB or other board surface area. The present invention can be used to advantage with CSP packages of a variety of sizes and configurations ranging from typical BGAs with footprints somewhat larger than the contained die to smaller packages such as, for example, die-sized packages such as DSBGA. Although the present invention is applied most frequently to chip scale packages that contain one die, it may be employed with chip scale packages that include more than one integrated circuit die.
In a two-high CSP stack or module devised in accordance with a preferred embodiment of the present invention, two CSPs are stacked, with one CSP disposed above the other. The two CSPs are connected with a pair of flex circuits. Each of the pair of flex circuits is partially wrapped about a respective opposite lateral edge of the lower CSP of the module. The flex circuit pair connects the upper and lower CSPs and provides a thermal and electrical path connection path between the module and an application environment such as a printed wiring board (PWB).
The present invention may be employed to advantage in numerous configurations and combinations of CSPs in modules provided for high-density memories or high capacity computing.